This is how I sometimes feed two babies at once. We are not on a super strict feeding schedule, so they usually eat about a half hour apart. But sometimes, they just can't wait for someone else to finish.
That's when I pull out these bottle proppers. These were handmade by a woman I found online while searching for bottle proppers. They are a u-shaped pillow with basically a "scrunchie" sewn on top to hold the bottle. The "arms" of the pillow go on either side of the face and the curved end rests on their chest. In these photos they are resting on the high chair for a better angle while sitting up.
They definitely help, but aren't perfect. They tip over, and sometimes the babies lose their latch on the bottle while eating. And they still need to be burped one at a time! A little help is enough though!